This application is related to Japanese Patent Applications No. 2000-167510 filed on Jun. 5, 2000, No. 2000-344206 filed on Nov. 10, 2000, and No. 2001-116959 filed on Apr. 16, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a heat-pump type hot-water supply system, and more particularly, relates to technique for restraining a decrease of boiling-up ability resulting from an increase in a water temperature in a final stage of boiling-up and for reducing running cost by optimizing a boiling-up terminating condition.
2. Description of Related Art
Applicants of the present application invented a hot-water supply system with a heat pump cycle which includes a refrigerant compressor, a refrigerant flow passage of a water heat exchanger, an expansion valve, an air heat exchanger provided with an outdoor fan, and a hot water circuit. In the hot water circuit, a hot-water storage tank for storing hot water for a hot-water supply and a hot-water flow passage of the water heat exchanger are connected through a hot-water pipe, and a pump is provided in the hot-water pipe between the hot-water storage tank and the hot-water flow passage. In addition, a difference between a refrigerant temperature at an outlet side of the refrigerant flow passage and a hot-water temperature at an inlet side of the hot-water flow passage is compared with a target temperature difference, and a valve opening degree of the expansion valve is controlled to improve a cycle efficiency (coefficient of performance, COP).
When the temperature of the hot-water within the hot-water circuit is high, it becomes difficult to cause a temperature difference between the refrigerant temperature on the outlet side of the refrigerant flow passage and the hot-water temperature on the inlet side of the hot-water flow passage, and the expansion valve is controlled in a direction that opens the expansion valve. As a result, a high-pressure side refrigerant pressure within the heat pump cycle becomes lower. Particularly, when temperature of hot-water within the hot-water storage tank has lowered by stopped operation for many hours and thereafter the hot-water is boiled up again, or when the temperature of hot-water within the hot-water circuit including the hot-water storage tank is ununiform, there arises a problem.
In addition, because the temperature of hot water in the hot water circuit increases with an elapse time of operation of the hot-water supply system, the temperature difference between the refrigerant temperature on the outlet side of the refrigerant flow passage and the hot-water temperature on the inlet side of the hot-water flow passage becomes smaller in a final stage of boiling up of hot water, and the control of the expansion valve due to the temperature difference becomes difficult.
In view of the foregoing problems, it is an object of the present invention to provide a hot-water supply system with a heat pump cycle, capable of preventing a decrease of hot-water supply ability due to a lowered refrigerant pressure within the heat pump cycle.
It is a second object of the present invention to provide a hot-water supply system with a heat pump cycle, capable of resolving insufficient ability and securing a target hot-water supply temperature during a boiling-up operation without adding any new functional components, and improving stability and reliability of a hot-water control in low cost.
A third object according to the present invention is to provide a hot-water supply system with a heat pump cycle, in which the boiling-up operation is performed in consideration of electric power rate per unit time based on a time-of-day rate system, and running cost is reduced.
It is a fourth object of the present invention to provide a hot-water supply system with a heat pump cycle, which accurately perform a control immediately before boiling-up.
According to an aspect of the present invention, in a hot-water supply system, a control unit controls operation of an expansion valve in a first direction increasing an opening degree of the expansion valve, when a temperature difference, between a refrigerant temperature at an outlet side of a refrigerant passage in a fluid heat exchanger and a fluid temperature at an inlet side of a fluid passage in the fluid heat exchanger, is smaller than a target temperature difference. On the other hand, the control unit controls operation of the expansion valve in a second direction decreasing the opening degree of the expansion valve, when the temperature difference is larger than the target temperature difference. In addition, when the temperature difference is smaller than the target temperature difference and the expansion valve is controlled in the first direction, the control unit controls a high-pressure side refrigerant pressure before being decompressed in the heat pump cycle to be equal to or higher than a predetermined refrigerant pressure in such a manner that the fluid has the target fluid temperature. Accordingly, it is possible to maintain the high-pressure side refrigerant pressure of the heat pump cycle to exceed the predetermined refrigerant pressure, and the target supply temperature of the fluid can be secured in the hot-water supply system. As a result, the hot-water supply system can prevent a decrease of hot-water supply ability due to a lowered refrigerant pressure within the heat pump cycle.
According to an another aspect of the present invention, in a hot-water supply system, when the control unit determines a heat radiation from the refrigerant to air in an air heat exchanger, the control unit controls a high-pressure side refrigerant pressure before being decompressed in the heat pump cycle to be equal to or higher than a predetermined refrigerant pressure at least until the heat radiation stops. Therefore, the high-pressure side refrigerant pressure in the heat pump cycle can be maintained at a pressure equal to or larger than the predetermined pressure, and the target hot-water supply temperature can be obtained without a heat loss when the fluid is water.
According to a further another aspect of the present invention, in a hot-water supply system, when the fluid temperature at the inlet side of the fluid passage in the fluid heat exchanger exceeds a predetermined temperature, the control unit sets a target pressure to an upper limit pressure that is lower than a compression resistance pressure of materials constituting the heat pump cycle, so that the high-pressure side refrigerant pressure of the heat pump cycle corresponds to the target pressure. Therefore, the high-pressure side refrigerant pressure in the heat pump cycle can be maintained at a pressure equal to or larger than the predetermined pressure, and the target hot-water supply temperature can be obtained when the fluid is water.
Preferably, the control unit presumes the high-pressure side refrigerant pressure based on the fluid temperature on the inlet side of the fluid passage in the fluid heat exchanger and the refrigerant temperature on the inlet side of an air heat exchanger, and increases or decreases the valve opening degree of the expansion valve in such a manner that this presumed refrigerant pressure corresponds to the upper limit pressure. Accordingly, when a relationship between the refrigerant temperature on the inlet side of the air heat exchanger and the high-pressure side refrigerant pressure is set in graphical or map based on for each fluid temperature on the inlet side of the fluid passage in the fluid heat exchanger, the high-pressure side refrigerant pressure will be presumed, based on the fluid temperature on the inlet side of the fluid passage in the fluid heat exchanger and refrigerant temperature on the inlet side of an air heat exchanger. Since there is no need to provide a high-pressure side refrigerant pressure sensor, cost for the component can be reduced and a maintenance for the high-pressure side refrigerant pressure sensor is not required.
Preferably, when the fluid temperature on the inlet side of the fluid flow passage exceeds the predetermined temperature, the high-pressure side refrigerant pressure of the heat pump cycle is controlled to correspond to the target pressure only in a midnight time zone which is cheapest in power rate. Thereby, the running cost of the hot-water supply system can be reduced.
According to a further another aspect of the present invention, in a hot-water supply system, the control unit has heat-storing amount calculating means for calculating a heat storage amount based on an amount of fluid in the tank and temperature of the fluid in the tank. Further, in a midnight time zone in which the power rate is set at the lowest, the control unit starts a boiling-up operation when the heat storage amount of hot water within the tank is smaller than a first predetermined amount, and stops the boiling-up operation when at least one of a first condition, where the temperature of the fluid on the inlet side of the fluid flow passage is higher than a first preset temperature, and a second condition, where the heat storage amount of hot water within the tank becomes larger than the first predetermined amount, is satisfied. In a morning and evening time zone in which the power rate is set comparatively low, the control unit starts the boiling-up operation when the heat storage amount of the fluid within the tank is smaller than a second predetermined amount, and stops the boiling-up operation when at least one of a third condition, where the temperature of fluid on the inlet side of the fluid flow passage exceeds a second preset temperature, a fourth condition where the heat storage amount of the fluid within the tank is larger than the second predetermined amount, is satisfied. In addition, in a daytime time zone in which the power rate is set high, the control unit starts the boiling-up operation when the heat storage amount of fluid within the tank is smaller than a third predetermined amount, and stops the boiling-up operation when at least one of a fifth condition, where the temperature of the fluid on the inlet side of the fluid flow passage exceeds a third preset temperature, and a second condition, where a heat storage amount of the fluid within the tank reaches the third predetermined amount, is satisfied. Accordingly, the hot-water supply system is capable of independently determining a start determining condition of the boiling-up operation of the fluid (e.g., water) and a termination determining condition of the boiling-up operation for each time zone in accordance with a pattern in which the user uses the fluid within the tank. As a result, the power rate can be made cheap while depletion of the supply fluid is not caused in all time zones. Thus, the running cost of the hot-water supply system can be reduced.
According to a further another aspect of the present invention, when temperature of the fluid at the inlet side of the fluid passage is lower than a control change temperature, the control unit controls the opening degree of the expansion valve so that a temperature difference between the refrigerant temperature at the outlet side of the refrigerant passage and the fluid temperature at the inlet side of the fluid passage is within a predetermined range. Further, when the temperature of the fluid at the inlet side of the fluid passage is equal to or higher than the control change temperature, while the control unit controls the capacity of the pump at a constant value, the control unit controls the opening degree of the expansion valve to be increased when the temperature of the fluid at an outlet side of the fluid passage is higher than a first set temperature, and controls the opening degree of the expansion valve to be decreased when the temperature of the fluid at the outlet side of the fluid passage is lower than a second set temperature lower than the first set temperature. Therefore, the hot-water supply system can accurately perform a control immediately before the boiling-up. Further, the hot-water supply system can resolve insufficient ability and secure a target fluid supply temperature during the boiling-up operation without adding any new functional components, and improve stability and reliability of the fluid temperature control in low cost.
According to a further another aspect of the present invention, when temperature of the fluid at the inlet side of the fluid passage is lower than a control change temperature, the control unit controls the opening degree of the expansion valve so that a temperature difference between the refrigerant temperature at the outlet side of the refrigerant passage and the fluid temperature at the inlet side of the fluid passage is within a predetermined range. On the other hand, when the temperature of the fluid at the inlet side of the fluid passage is equal to or higher than the control change temperature, the control unit controls the opening degree of the expansion valve to be decreased when a refrigerant temperature discharged from an outlet of the compressor is equal to or lower than a first refrigerant set temperature, and controls the opening degree of the expansion valve to be increased when the refrigerant temperature discharged from the outlet of the compressor is equal to or higher than a second refrigerant set temperature set higher than the first refrigerant set temperature. In addition, the control unit controls the capacity of the pump to be increased when the temperature of the fluid at the outlet side of the fluid passage is equal to or higher than a first fluid set temperature, and controls the capacity of the pump to be decreased when the temperature of the fluid at the outlet side of the fluid passage is equal to or lower than a second fluid set temperature lower than the first fluid set temperature. Accordingly, the hot-water supply system can accurately perform a control immediately before the boiling-up.